This invention relates to cushioning materials for dynamic impact applications, such as energy-absorbing materials for automobiles and other vehicles.
Polymer foams are widely used in a variety of cushioning applications. Soft polyurethane foams are commonly used in pillows, seating, mattresses and similar applications where softness and comfort are predominating factors. Thermoplastic foams such as closed-celled extruded polyolefin foams are prevalent in a wide range of packaging applications.
In most cushioning and packaging applications, the foam material is usually designed to withstand low to moderate compressive stresses. The foam is designed such that under these conditions, the strain induced in the foam as a result of the applied compressive stress during normal use is within the so-called elastic limit of the foam. Within the elastic limit, the induced strain (i.e. compression of the foam) is approximately proportional to the applied compressive stress, so that, for example, doubling the stress will induce approximately a doubling of the strain. In addition, foam that is compressed within its elastic limit will return, when the compressive force is removed, approximately the same amount of energy as was required to compress the foam. This allows the foam to absorb energy from low-to-moderate level impacts without permanently deforming the foam or significantly diminishing its ability to cushion further impact events of like magnitude. For many commonly used polymeric packaging foams, the elastic limit is exceeded when a strain of about 3-10% of the original foam thickness is exceeded.
In certain other cushioning applications, the cushion is intended to dissipate much higher levels of energy. Applications of this type notably include energy-absorbing automotive members that are intended to prevent or minimize injury to vehicle occupants in an accident. Many automotive interior parts include this sort of cushioning, including knee bolsters, instrument panels, headliners, roof pillars and doors. Quite often, these energy-absorbing cushions are designed to be used in conjunction with other energy management features, such as frontal air bags or side curtain air bags. Unlike cushions used in seating or most packaging applications, cushions used in these applications are designed to absorb high levels of energy at high strain rates. Such conditions exceed the elastic limits of the cushion, permanently deforming it in order to dissipate energy and reduce injury.
The severity of personal injuries in automotive accidents is often a result of the maximum deceleration experienced as an occupant makes contact with an automobile component. This maximum deceleration can be reduced in two ways. First, it can be reduced by lengthening the time period over which the deceleration occurs. Second, the maximum deceleration can be reduced if the energy of impact is dissipated more uniformly over that longer time period. A cushioning foam, therefore, desirably continues to absorb energy at a more or less constant rate as it is compressed to a fraction of its original thickness.
The behavior of most cellular polymers is such that the compressive stress needed to induce strain increases more or less linearly up to the elastic limit, i.e., to a strain of about 3-10% or so of the original foam thickness. After exceeding the elastic limit, the compressive stress tends to remain nearly constant up to about 20 or 30% strain, and then increases dramatically as more strain is induced to the foam. It would be more desirable if the compressive stress remains nearly constant to higher strains, such as 40-60% strain or more. This would both lengthen the time over which deceleration occurs (by distributing energy over the longer time period needed to compress the cellular polymer to the higher strain) and reduce the maximum deceleration because energy is absorbed more evenly as the cellular polymer is compressed.
An anisotropic form of polymeric foam, sometimes known as a coalesced strand foam, has been used in these applications. This foam, which is sold under the trade name Strandfoam® by The Dow Chemical Company, exhibits significantly higher compressive strength in the direction of extrusion than in orthogonal directions. This anisotropic behavior is believed to be due in part to the particular method by which the foam is made. Small diameter “strands” of a foamable resin mixture are separately extruded, and the extrudates are brought together before they cool to form a larger composite that is made up of a large number of separate strands. This coalesced strand foam performs well in dynamic impact applications, but has the drawback of being somewhat expensive. Foams of this type that have been used in dynamic impact applications have been higher density materials, which further increases cost. A further problem with these foams is that the direction of highest compressive strength is in the direction of extrusion. As most energy-absorbing cushions are rather thin in the direction of expected impact, this means that these anisotropic foams must be cut into thin slices to be used effectively. This adds fabrication costs and leads to excessive waste. This also limits the cross-sectional area of the energy-absorbing member to the cross-sectional areas of the foam as extruded, unless still further costs are incurred to assemble foam pieces into a larger cross-section.
DE 44089298 A1 describes highly anisotropic polyethersulfone foam as being useful as a shock absorbing element in helmet applications. This foam is said to have a density of 50 kg/m3 or more and at that density is said to have a compressive strength of 600 kPa. This foam has extremely large and elongated cells that have a length/diameter ratio of about 10 and a diameter (smallest dimension) of about 0.8 mm.
As a result, it would be desirable to provide a cushion that is relatively inexpensive, and performs well in dynamic impact applications.